It is known that electromagnetic fields can be used for thawing, warming and treating different loads consisting of dielectric materials. Examples of such dielectric materials are proteins, wood pulp, alcohols and salts dissolved in water. Examples of electromagnetic fields include microwaves, (frequencies above 900 MHz) and radio fields (frequencies below 900 Mhz). There are many examples of demanding medical and industrial applications requiring a fast and homogenous warming (i.e., even field distribution). One example is a bag with 250 ml frozen blood plasma intended for transfusion, another example is a bag of frozen stem cells, but it can also be about controlling different chemical processes such as the acetylating of wood.
A common problem of warming with microwaves is that the wavelength is short, at 2500 MHz, (the commercial microwave frequency) the wavelength is 12 cm in vacuum/air and in most dielectric loads the wavelength is 2-5 cm. At frequencies used regularly in microwave products hotspots are common due to reflection and interference. A high frequency will also result in development of superficial energy.
The energy development in a dielectric material is determined by following relationship.W=∈′*tan(δ)*f*E2 
Wherein W is the power, ∈′ is the constant of dielectricity, tan(δ) is the loss factor, f is the frequency and E is the field strength.
A measure of the energy distribution is the penetration of depth which is defined by δ=c/π*f√∈*tan(δ) where c is the speed of light in vacuum.
In order to avoid hotspots as a result of reflection and interference and obtain a more homogenous warming process, a lower frequency can be applied. The wavelength in the load increases, as a result, the homogeneity of the warming process improves and the problems with so called hot spots are reduced and possibly eliminated.
If a longer wavelength (lower frequency) is applied, at unmodified power and dielectricity values, the field strength will increase compared with higher frequency. The power generation is a function of E2, therefore a relatively small increase of the field strength at a lower frequency will result in a considerable increase of the power generation compared with higher frequencies.
At transitions between load and surrounding air; at corners/edges and protrusive parts, an increase of the field strength with correlating heat generation often appear. It depends on the wavelength: the longer the waves are, the easier the field lines will turn around corners/edges and protrusive parts with a correlating increase in field strength.
Warming a load with electromagnetic fields without any local overheating requires that the wavelength is long enough in relation to reflection and interference phenomena and that the turning around corners/edges and protrusive parts is reduced or preferably eliminated. It is favorable if this can be done without considerable energy losses.
At frequencies below 900 MHz the probability for distinct hotspots is reduced and at frequencies below 300 MHz it is negligible. At shorter wavelengths the energy is more concentrated at the extinction points compared with longer wavelengths. This is especially valid if the load has a large constant of dielectricity that will additionally shorten the wavelength. However, at longer wavelengths the problems with overheating increase due to the turning of the electromagnetic fields/field lines at corners, edges and protrusive parts.
In order to solve the problem with turning of field lines at protrusive parts different solutions have been suggested.
In patent UK 599,935 a dielectric load is placed in a liquid with the same constant of dielectricity and loss factor as the load that is being heated. If there is enough of the surrounding liquid local overheating is eliminated on/in the load. The disadvantage with this solution is that the major part of the energy is absorbed by the surrounding liquid resulting in a negative energy aspect. Further a controlled and repetitive warming process of a dielectric load is made more difficult because the temperature of the surrounding liquid is altered/changed due to accumulated energy absorption contributing to the warming of the load.
In patent WO 02/054833 the dielectric load is surrounded with a dielectric material having a dielectric constant similar to the dielectric constant of the load but the loss factor of the surrounding material is small compared with the loss factor of the load. In this patent the load of a blood fraction, for example frozen blood plasma intended for transfusion is stored in a PVC bag. In that way, the turning of the electromagnetic fields/field lines at corners, edges and protrusive parts is reduced as well as no energy is absorbed in the surrounding material.
It is difficult to obtain a solution with identical dielectric constants of the load and the surrounding material. Pockets of air may also appear between the load and the surrounding material/liquid that may cause concentration of the electric/electromagnetic field to certain areas that will result in parts of the dielectric load being warmer than others.
If the load consists of perishable materials such as blood fractions intended for transfusion local overheating can result in devastating consequences.
Other biological materials such as frozen stem cells, organs intended for transfusion, etc. for the same reasons, require a homogenous thawing/warming process.
There are other applications that will benefit from a fast and homogenous thawing and warming process. One application is frozen fish and meat used as raw material in food processing industry. These raw materials are usually stored in frozen 10 kg blocks and have to be thawed before processing. Because of hygienic reasons, the surface has to be kept cold; therefore such blocks of fish and meat are thawed slowly in cold-storage rooms. The slow thawing process generates considerable capital costs and requires considerable planning efforts in order to achieve a cost efficient production. The thawing of raw materials of fish and meat is costly.